objects do not render
In the file you posted, the only issue that I saw was that the camera was not pointing at the objects, so it would just render blank space. I reset the camera to point at the objects and it rendered just fine.

As an aside, I believe metaballs cannot be joined unless you convert them to a mesh first. I selected one, pressed Alt+C and chose mesh from option and it joined them all. It does make them permanent however, and you can't move them around individually anymore.